Endoscope systems for observing a body cavity of a patient are known. In general, an endoscope system is provided with a light source optical system for illuminating a body cavity of a patient. A concrete configuration of the light source optical system of this type is described, for example, in Japanese Patent Provisional Publication No. 2002-333533A (hereafter, referred to as patent document 1).
The light source optical system described in the patent document 1 includes a light source side lens (a convex lens or a ball lens) for letting light emitted from a light source enter a light guide. The light guide is an optical fiber including a core part and a clad part, and is configured such that the core diameter thereof is enlarged in a cone shape on an entrance end face side. Since, in the patent document 1, the area of an entrance end face of the light guide is large, a large amount of light can be taken in the light guide and thereby the inside of a body cavity can be illuminated brightly.